chi (n.)

22nd letter of the Greek alphabet, representing a -kh- sound (see ch). The letter is shaped like an X, and so the Greek letter name was used figuratively to signify such a shape or arrangement (as in khiasma "two things placed crosswise;" khiastos "arranged diagonally; marked with an X;" khiazein "to mark with an 'X', to write the letter 'X'"). Some dialects used chi to represent the -ks- sound properly belonging to xi; Latin picked this up and the sound value of chi in Latin-derived alphabets is now that of English X.